This invention relates to a process for preparing grafted organic polyester of phosphoric acid. The grafted organic polyester of phosphoric acid, incorporated into water-borne coating compositions for automobile, containing metallic flake pigments, may prevent or reduce the tendency of metallic flake pigments to react with the aqueous phase of the compositions, resulting in generation of gas.
Corporating metallic pigments into coating compositions is well known, and, particularly it is an established practice to use aluminum flake pigments in compositions intended for production of the so-called "glamour" finishes upon automobile bodies, by which a differential light refraction effect, depending on the angle at which the coated surface is viewed, is achieved.
Usually, such coating compositions were solvent-borne, but in recent years, as a result of the growing concerns for reduction of atmospheric pollution, a trend towards water-borne compositions has increased. For example, there is described in British Patent Specification No. 2073609A, a coating process in which a specified type of water-borne basecoat composition containing aluminum flake pigment was applied for the first time.
Difficulties are confronted in incorporating metallic pigments, particularly those of aluminum or aluminum alloys, into water-borne coating compositions, caused by the tendency of the pigment to react with the water present, resulting in generation of hydrogen. This could cause a safety problem if the compositions are stored in closed containers, and also a degeneration of coating product by generation of hydrogen.
Many solutions have been proposed in which this "gassing" effect can be minimized or prevented, most of which involve some form of chemical treatment of metallic pigments to render them less reactive towards the aqueous medium of coating composition. It is known that orthophosphoric acid is most effective for this purpose, but its presence in coating compositions leads to poor chemical properties in films derived from them. Alkyd esters of phosphoric acid make up for this problem to a limited extent, but its addition to a basecoat causes an impaired adhesion with topcoat. It has now been proposed to use organic ester of phosphoric acid, but it is shown that storage problem persists and poor mechanical properties cannot be overcome without effective preparation of Water-soluble aluminum.